In Minecraft, zombies drop feathers, and feathers are used for making arrows. Normally, you have to go out and kill zombies or chickens manually to get them. What this guy did was make a trap where zombies spawn and fall into flowing water. The water flows to a field of cacti, where the zombies get stuck and die (cacti hurt you to touch them). Some of the zombies drop feathers, which continue flowing with the water to an opening. In effect, the guy gets feathers delivered to him automatically, with no effort.

You can rig up similar contraptions for other resources, like a "bacon farm" which delivers pork (pork heals you).

Check it out: http://www.youtube.com/watch?v=55ZHfg80m_Q

The basic idea is this: There's a block that spawns enemies. You get items by killing monsters. But killing monsters is dangerous. So... if we can automatically make something that kills monsters, we're in business.

So you carve a room out around the monster spawner, and then make the room deadly. Problem is, then you can't go inside to collect the items they drop. So you make the room covered in flowing water, which drags the items downstream for collection.

Feathers are used to build arrows, water is just used to transport feathers.

Arrows can be used with a bow to fight monsters.